1. Field of the Invention
This invention relates to a laminated transparency and more particularly to a moisture seal for an aircraft windshield to prevent delamination of the windshield plies due to moisture absorbed by the windshield interlayers.
2a. Technical Considerations
Aircraft transparencies are typically a laminated assembly including a plurality of interlayer material plies interposed between transparent rigid plies. The rigid plies can be glass or any other well known substitute such as polycarbonates, acrylic resins, polyesters, and rigid transparent polyurethanes. The thermoplastic interlayers, which bond adjacent rigid plies together to form a unified laminated assembly, may be a material such as polyvinyl butyral, silicone, or urethane. When exposed to moisture over a period of time, these interlayer materials have been observed to lose their adhesive properties and cause degradation of the bond between the rigid plies resulting in reduced strength and possible delamination of the transparency. In order to prevent the penetration of moisture into the interlayer from the periphery of the transparency, a moisture seal is applied along the edge of the transparency or along any other portion of the transparency where the interlayer material is exposed to moisture. Besides sealing the interlayer material from moisture, the sealing material must also by flexible at the low temperatures to which the aircraft transparency is exposed so as not to crack the seal or transparency or pull of any edge chips from the periphery of the transparency. In addition, depending on the application, the sealing material must also be able to maintain its moisture sealing properties under prolonged exposure to ultraviolet radiation which can degrade a sealant material.
2b. Patents of Interest
U.S. Pat. No. 4,046,951 to Stefanik teaches a laminated transparency with a composite edge sealing arrangement consisting essentially of a thin barrier layer of an organofluoro composition and a sealant layer having moisture resistant properties that will bond to the lamina of the transparency and the barrier layer.
U.S. Pat. No. 4,121,014 to Shaffer teaches a transparent laminate with a plasticized polyvinyl acetal ply secured to a polycarbonate ply by an adhesive film of polyurethane or epoxy resin.
U.S. Pat. No. 4,204,374 to Olson teaches an edge design for an impact resistant windshield. A bevel on the surface of the outboard ply about the windshield's perimeter is filled with a flexible sealant such as polyurethane, polysulfide, silicone, or the like to protect against delamination.
U.S. Pat. No. 4,277,294 to Orcutt and 4,324,373 to Zibritosky teach a laminated window panel having a silicone bumper strip about its periphery to provide a barrier against moisture penetrating the interlayer material.
U.S. Pat. No. 4,284,677 to Herliczek teaches a glazing unit which uses polyvinyl butyral as interlayer material and an edge seal in a glass laminate.